1. Field of the Invention
The present invention relates to a mobile communication device with a positioning capability. In particular, the present invention relates to a mobile communication device (e.g., a cellular telephone) that is also capable of receiving a global positioning system (GPS) signal, and which shares an oscillator between the communication and positioning functions.
2. Description of the Related Art
The utility of a mobile communication device (e.g., a cellular telephone) is enhanced if it is provided the additional capability of receiving and processing global positioning system (GPS) signals that can be used to determine the position of the mobile communication device.
To provide for both positioning and communication functions, it is possible to share a local oscillator between the receiver and transmitter of the communication circuit and the GPS signal receiver. While sharing a local oscillator can reduce the cost and bulkiness of such a mobile communication device, there are some practical problems to be overcome to achieve high performance. For example, in cellular communications, when a mobile communication device leaves the service area of a base station and enters into the service area of another base station, a xe2x80x9chand-offxe2x80x9d procedure takes place in which the mobile communication device tunes into the operating frequency or channel of the new base station. During the hand-off, it is often necessary to adjust the offset (i.e., deviation from the base station""s xe2x80x9cnominal center frequencyxe2x80x9d), as each base station may have a different offset. In degraded signal conditions, continuous tracking of a carrier may also require an offset frequency adjustment. However, if such an adjustment is made during the acquisition of a GPS signal, both the mixing frequency and the sampling frequency of the GPS receiverxe2x80x94used in down-converting and digitizing the GPS signal, respectivelyxe2x80x94are affected. The received signal may yield an erroneous result, or even a failure to detect the GPS signal. In fact, in one system, a 0.05 parts-per-million (ppm) adjustment has the effect of a 79 Hz shift in the carrier frequency in the received GPS signal.
One approach avoids the corruption of the GPS signal by locking the communication circuit out from accessing the oscillator for frequency adjustment so long as a GPS signal acquisition is in process. However, such an approach is undesirable because it prevents the mobile communication device from establishing contact with one or more base stations while a GPS signal is being acquired, which may lead to temporary loss of communication service. Also, such an approach complicates the control software in the mobile communication device, thereby deterring manufacturers from incorporating positioning capability in their mobile communication devices.
In GPS signal detection, one source of uncertainty in the carrier modulation frequency in the received signal is the xe2x80x9cclock Doppler,xe2x80x9d which results from the unknown syntony between the clock on the signal source (e.g., a GPS satellite) and the receiver""s own clock. Precise knowledge of the local oscillator""s frequency can reduce the frequency search space (xe2x80x9cDoppler rangexe2x80x9d) for the GPS signal. At any given time, the actual frequency of a local oscillator depends on a number of variables, such as manufacturing variations, temperature, aging and operating voltage. Oscillators used in signal sources (e.g., GPS satellites) are typically well-characterized and are tuned to the specified frequency with high accuracy. Because of their cost, high power requirements, and bulkiness, however, such oscillators are unsuited for use in a mobile communication device. To more accurately determine the operating frequency of a local oscillator, the prior art typically requires a more costly oscillator then conventionally found in a mobile communication device. Others require a complex calibration procedure to tune the oscillator to a precision carrier frequency. The latter approach is disclosed, for example, in U.S. Pat. No. 5,874,914 to Krasner, entitled xe2x80x9cGPS Receiver utilizing a Communication Link.xe2x80x9d Neither approach is satisfactory from a cost and performance standpoint.
According to one embodiment of the present invention, provided in a mobile communication device, is a method for compensating for a frequency adjustment in an oscillator shared between a communication circuit and a positioning signal receiver. In one embodiment, the method includes (a) at a first point in time, beginning receiving and storing into a storage device the positioning signal; (b) at a second time point, adjusting a frequency of the oscillator by a given amount; (c) recording the frequency adjustment; (d) at a third time point, completing receiving and storing of the positioning signal from the positioning signal receiver; and (e) processing the positioning signal, taking into consideration the frequency adjustment. In one implementation, the second time point is recorded as the time at which the frequency adjustment of the oscillator is made. Having the knowledge of the time at which the frequency adjustment is made, the processing searches for a frequency shift in the received positioning signal between the second time and the third time. In another implementation, the amount by which the frequency of the oscillator is adjusted is recorded, and the processing searches for a time point at which the frequency adjustment of the oscillator is made. In one implementation, the processing integrates a correlation function.
The present invention is applicable to GPS processing using aiding data, such as satellite ephemeris data. The present invention is particularly applicable to cellular communication in which an oscillator adjustment may be made when the mobile receiver moves between service areas of base stations.
Thus, accurate processing of the positioning data is ascertained without preventing the communication circuit from accessing the shared oscillator while positioning data is being acquired.
According to another aspect of the present invention, a mobile communication device determines an operating frequency of an oscillator based on a reference signal from a reliable time base. In one embodiment, a beginning time point of the reference signal is received by the mobile communication device. When the beginning time point of the reference signal is detected, a counter is enabled to count a number of cycles in a clock signal derived from the oscillator. The ending time point of the reference signal is then detected. Upon detecting the ending time point of the reference signal, the counter is stopped to prevent the counter from further counting. Finally, the frequency of the oscillator is determined based on the count in the counter and an expected time that elapsed between the beginning time point and the ending time point.
The present invention can use reference signals having a known duration in time, or having recurring events in the reference signal that recurs at a fixed frequency. In some implementation, the frequency of the oscillator so derived can be further adjusted, taking into account the processing times in the mobile communication device for detecting the beginning time point and the ending time point.
Using the method of the present invention, the operating frequency of a local oscillator can be determined to the accuracy of the oscillator of the base station oscillator, without incurring the expense or inconvenient bulkiness of the more costly, higher precision oscillator typically found in base stations or less mobile equipment. In a GPS signal receiver, by removing the uncertainty in oscillator frequency, the Doppler range over which the positioning signal receiver software searches can be further limited.
The present invention is better understood upon consideration of the detailed description below and the accompanying drawings.